1. Field of the Invention
The invention generally relates to optical image projection systems and is more particularly concerned with the wide-angle anamorphic display systems employing adjustable mirrors for forming a compact and adjustable optical projection path.
2. Description of the Prior Art
Generally, individual prior art optical forcussing elements such as lenses or mirrors are devices of fixed shape and of fixed focal length and have in themselves no inherent characteristic conveniently permitting variation of focal length over useful ranges. Such fixed optical devices in large sizes are expensive to manufacture, requiring molding or casting processes with precision finishing operations of grinding and polishing. It is also known to attempt to attain focal length variability in optical elements by the use of a thin reflective flexible diaphram whose shape is controlled by fluid pressure. For example in the Kopito U.S. Pat. No. 3,031,928 for "Control of Flexible Surfaces by Means of a Probe", a flexible membrane defines a circularly symmetric active surface of an optical element and forms a curved boundary between adjacent pressure chambers, at least one chamber having a transparent end window. The deflection of the membrane appears to be determined by the differential pressure across the membrane so that a capacity proximity probe may constantly measure its deflection. Any positional error is corrected by a complex system of servos controlling relative gas flow with respect to the chambers as separated by the membrane which, if it is to act as a mirror for light entering the end window, is coated with a reflecting material. It will be readily apparent to those skilled in the art that the flexible membrane is of delicate nature; for that reason and because differentially variable gas pressures must be employed, an expensive container must be provided for the membrane. Gas must be provided and electrical power must be continuously supplied to put the gas under pressure, to sense diaphragm position, and to operate the servo systems. Thus, the device is undesirably complex, is initially expensive, and continuously requires expensive electrical power in operation.
The double convex or double concave variable focus lens ideas of the De Luca U.S. Pat. No. 3,161,718 for a "Variable Power Fluid Lens" similarly operate by virtue of the application of fluids under pressure at the sides of transparent membranes or diaphragms and, like the Kopito device, vary the actual tension to which each membrane is subjected. In both concepts, variation of external conditions, particularly of temperature in the Kopito device, will often produce differential heating effects, causing excessive working of the servos or controls, subjecting them to undesired wear and wasting electrical power. Undesirably complex controls are required for both concepts that are to be avoided when a simple adjustable-focus mirror is desired, controls that render them unattractive for all but perhaps specialized purposes where initial and operating costs are of no large consequence.
In devising an optical projection system employing the present invention, it is recognized that simple, low-cost adjustable-focus optical elements are preferred, as the projection system should be constructed around a low-cost anamorphic optical system with a space-saving folded optical path and should operate as a short throw, wide-angle image projection system in the interest of compactness.
Conventional projectors in universal use have maximum image beam width angles of only about 30.degree.. It is well known that such image beam angles require the projector to be placed approximately twice the distance from the screen as the actual width of the projected image on the screen. For many applications, as in navigation trainers, such throw distances are excessive; for that reason, the projectors themselves have been directly equipped with expensive wide-angle projection lens units. These lens units, because of their complexity, also suffer from substantial light losses, light fall-off at their edges, and chromatic aberration effects. Such conventional projection systems are additionally not found to be readily adaptable to producing anamorphic displays; that is, display images in which the width-to-height ratio can be conveniently modified. For example, in marine navigation, the most important visual data affecting navigation accuracy and safety is information about the azimuth location of potentially dangerous objects; the visual elevation information is of somewhat secondary nature. By magnifying apparent azimuth displacements and consequently azimuth velocities of objects seen on the display, the training value of the display is enhanced. In other applications, the height of the display may be the exaggerated dimension, as in an instrument landing trainer for aircraft. A satisfactory compact system for achieving an anamorphic display system is not available in the prior art.